A recent email was sent to fansites, and now this information is also available on StarCraft 2's site, in relations to cheating.

Blizzard Entertainment has always taken cheating in any form in Blizzard games very seriously, and that's no different for StarCraft II. If a StarCraft II player is found to be cheating or using hacks or modifications in any form, then as outlined in our end user license agreement, that player can be permanently banned from the game. This means that the player will be permanently unable to log in to Battle.net to play StarCraft II with his or her account.

Playing StarCraft II legitimately means playing with an unaltered game client. Doing otherwise violates our policies for Battle.net, and it goes against the spirit of fair play that all of our games are based on. We strongly recommend that you avoid using any hacks, cheats, or exploits. Suspensions and bans of players that have used or start using cheats and hacks will begin in the near future.

NethaeraCommunity ManagerBlizzard Entertainment

We'd like to point out that as of this moment, this does not exclude simply edits such as audio files, backgrounds, texture maps, etc from being considered "cheating" if you are editing any MPQ format file. For now, we highly recommend against any true modding if the files are not locally within a map itself.